Four-color printing on paper in an offset rotary press is based on the principle of superposing, on the paper, four printed images each of which is in a primary color (black, blue, yellow, red). The paper runs under four print rolls, each of which carries a plate or the like fixed to the roll in a disposition which iu accurately determined and prints an image on the paper in one of the primary colors. All four images must be exactly superposed in order to ensure the colors are in register relative to one another. The print rolls are equipped with motors for displacing them in two perpendicular directions (rotary displacement about the roll axis, and transversal displacement along said axis), with the positions of the rolls generally being adjusted and trimmed relative to the black ink print roll.
When performing simultaneous recto-verso printing, the paper runs through successive groups of pairs of print rolls which are both associated with the same primary color, with the theory of printing on each face of the paper being the same as explained above. When very high quality printing is required, it is desirable to ensure that the frames of the images printed on both sides of the paper are in register relative to each other through the paper, and in general this requires the position of the roll for printing the black color on the verso face to be adjusted relative to the position of the roll for printing the same color on the recto face.
Methods and devices for automatically adjusting color superposition are already known in which a visible mark is printed on the paper by each roll in the vicinity of the printed image in a zone which will be removed when the paper is cut on leaving the rotary press.
Means are provided for successively detecting the positions of these marks on the paper, for determining the errors in the positions of these marks relative to one another, and for producing control signals for the print roll displacement motors tending to reduce and eliminate the positioning errors of the marks, thereby obtaining exact superposition of the colors. However, the marks formed on the paper are of relatively large size, and this means that they are not always completely eliminated when the paper is cut. Further, the accuracy with which the marks can be detected is relatively limited. Finally, known methods and devices lead to the path of the paper being modified by the addition of extra rolls.
Also, it is important for the time taken by the various print roll position adjustments for obtaining good color superposition to be as small as possible. These adjustments are performed when the rotary press is rotating, and since the paper passes through the press at high speed (5 to 10 meters per second) the quantity of paper wasted during these adjustments is large and expensive.
Accordingly, it is the object of the present invention to reduce these various drawbacks to a considerable extent, by virtue of a method and a device enabling print rolls to be positioned relative to one another rapidly and accurately.
It is a particular object of the invention to reduce the time taken for adjusting the superposition of color in an offset rotary press.
It is a further particular object of the invention to increase the accuracy of adjustment of print rolls in an offset rotary press.
It is a still further object of the invention to reduce the quantity of paper wasted during the adjustment process of print rolls in an offset rotary press.